Thin magnet alloy belt and resin bonded magnet

ABSTRACT

In order to secure stable magnetic properties in a magnet alloy ribbon obtained by a melt rapid cooling method, and obtain excellent magnetic properties and corrosion resistance in a bonded magnet, the area ratio of dimple-like recesses ( 22 ) present in the surface (roll surface) of the alloy ribbon in contact with a cooling roll during solidification is defined. As a result, an alloy ribbon for a magnet having stable magnetic properties can be obtained. The use of a powder obtained by grinding such an alloy ribbon enables formation of a bonded magnet having excellent magnetic properties and corrosion resistance.

TECHNICAL FIELD

The present invention relates to a magnet alloy ribbon, and particularlyto a rare earth permanent magnet alloy ribbon, and a resin bonded magnetusing a magnet powder obtained from the alloy ribbon.

BACKGROUND ART

In regard to a method of producing an alloy ribbon by jetting an alloymelt of a rare earth magnet material on a single metallic roll, JapaneseExamined Patent Publication No. 3-52528 discloses in line 30 of column 7on page 4 to line 42 of column 9 on page 5 that an alloy ingot sample isplaced in a quartz tube and melted, and then the melt is jetted at aconstant speed on a metallic disk having too high heat capacity for themelt through a circular orifice provided in the lower portion of thequartz tube to obtain an alloy ribbon. Japanese Unexamined PatentPublication No. 59-64739 reports that for a rare earth-transitionmetal-B system magnet composition, the rotational speed of a roll is animportant factor which influences the magnetic properties of an alloyribbon.

However, consideration has not been given to how the detaileddimensions, shape and surface state of an alloy ribbon affect magneticproperties.

In addition, a permanent magnet material produced by a conventionalrapid cooling method has the following problems.

1) Magnetic properties deteriorate due to variations in the microstructure which constitutes the alloy ribbon.

2) In the formation of a bonded magnet, when a resin is nonuniformlyadhered to a magnet powder, reliability, particularly corrosionresistance, deteriorates.

SUMMARY OF INVENTION

The present invention has been achieved for solving the problems of aconventional technique. In consideration of the surface state of thesurface (roll surface) in contact with a roll for mainly cooling analloy ribbon, a first object of the present invention is to provide analloy ribbon having excellent magnet characteristics.

A second object of the present invention is to provide a resin bondedmagnet having excellent magnetic characteristics and reliability, andformed by bonding a resin and a powder produced by grinding the alloyribbon as it is or after heat treatment.

In order to achieve these objects, a magnet alloy ribbon of the presentinvention is obtained by jetting a R-TM-B system (R is a rare earthelement such as Nd or Pr, and TM is a transition metal) alloy melt on arotating metallic roll to rapidly solidify the alloy melt, wherein thetotal area ratio of dimple-like recesses after solidification, which arepresent in the surface (roll surface) of the ribbon in contact with theroll during solidification, is 3 to 25%.

A magnet alloy ribbon of the present invention is obtained by jetting aR-TM-B system (R is a rare earth element such as Nd or Pr, and TM is atransition metal) alloy melt on a rotating metallic roll to rapidlysolidify the alloy melt, wherein the total area ratio of dimple-likerecesses, each of which has an area of 2000 μm² or more and which arepresent in the surface (roll surface) of the ribbon in contact with theroll during solidification, is 0 to 5%.

A magnet alloy ribbon of the present invention is obtained by jetting aR-TM-B system (R is a rare earth element such as Nd or Pr, and TM is atransition metal) alloy melt on a rotating metallic roll to rapidlysolidify the alloy melt, wherein the d/t ratio of the average depth (d)of dimple-like recesses to the average thickness (t) of the alloy ribbonafter solidification, which recesses are present in the surface (rollsurface) of the ribbon in contact with the roll during solidification,is 0.1 to 0.5.

A resin bonded magnet of the present invention is formed by grinding amagnet alloy ribbon as it is or after heat treatment, which is obtainedby jetting a R-TM-B system (R is a rare earth element such as Nd or Pr,and TM is a transition metal) alloy melt on a rotating metallic roll torapidly solidify the alloy melt, to form a powder; mixing thethus-obtained power and a resin; and then molding the mixture; whereinthe total area ratio of dimple-like recesses after solidification, whichare present in the surface (roll surface) of the ribbon in contact withthe roll during solidification, is 3 to 25%.

A resin bonded magnet of the present invention is formed by grinding amagnet alloy ribbon as it is or after heat treatment, which is obtainedby jetting a R-TM-B system (R is a rare earth element such as Nd or Pr,and TM is a transition metal) alloy melt on a rotating metallic roll torapidly solidify the alloy melt, to form a powder; mixing thethus-obtained powder and a resin; and then molding the mixture; whereinthe total area ratio of dimple-like recesses, each of which has an areaof 2000 μm² or more and which are present in the surface (roll surface)of the ribbon in contact with the roll during solidification, is 0 to5%.

A resin bonded magnet of the present invention is formed by grinding amagnet alloy ribbon as it is or after heat treatment, which is obtainedby jetting a R-TM-B system (R is a rare earth element such as Nd or Pr,and TM is a transition metal) alloy melt on a rotating metallic roll torapidly solidify the alloy melt, to form a powder; mixing thethus-obtained powder and a resin; and then molding the mixture; whereinthe d/t ratio of the average depth (d) of dimple-like recesses to theaverage thickness (t) of the alloy ribbon after solidification, whichrecesses are present in the surface (roll surface) of the ribbon incontact with the roll during solidification, is 0.1 to 0.5.

In accordance with claims the present invention, the surface state ofthe surface (roll surface) of the magnet alloy ribbon which contacts theroll, particularly the area ratio of dimple-like recesses present in thesurface, is defined to provide an alloy ribbon having excellent magnetproperties.

In accordance with therefor of the present invention, the thus-obtainedalloy ribbon is ground as it is or after heat treatment to form apowder, and the thus-obtained powder is mixed with a resin and thenmolded to provide a resin bonded magnet having excellent magneticproperties and reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an apparatus for producing a magnetalloy ribbon.

FIG. 2 is a schematic drawing showing the state of a magnet alloyribbon.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below.

1) Outlines of production method (magnet alloy ribbon and resin bondedmagnet)

FIG. 1 is a schematic drawing of an apparatus (super rapid coolingmethod) for producing a magnet alloy ribbon 15 by using a single roll14. This apparatus is installed in a chamber which can be evacuated.Schematically, a current is passed through a radio frequency heatingcoil 13 wound on a nozzle 12, which is filled with a raw material or amaster alloy in an inert atmosphere, to melt the raw material or masteralloy by induced electric current, to obtain an alloy melt 11. Heatingmeans is not limited to radio frequency heating, and a method comprisingproviding a heating element such as a carbon heater or the like on theperiphery of the nozzle may be used. Then, the melt is jetted on ametallic single roll which is set directly below a crucible and which isrotated about an axis 16 at a high speed, through an orifice (opening)provided at the bottom of the nozzle. Since the metallic roll has a highheat capacity for the jetted melt, the melt is solidified on the roll17, as well as being extended in the rotational direction of the roll toform an alloy ribbon. Each of terms will be described in further detailbelow.

The nozzle may be filled with each raw material metal which is weighedso as to have the desired composition (R-TM-B system) or a sample whichis cut off from a master alloy ingot previously produced in a radiofrequency melting furnace and having the desired composition. Althoughthe nozzle is preferably made of a quartz material, other ceramicmaterials such as high-heat-resistant alumina and magnesia, and the likemay be used. The orifice (opening) preferably comprises a circular holeor a slit. However, in the case of a slit, the length direction of theslit is preferably as close to the direction (width direction of theribbon) 24 perpendicular to the rotational direction of the roll aspossible.

The metallic roll is preferably made of a material such as a copperalloy, an iron alloy, chromium, molybdenum, or the like in order toobtain sufficient heat conductivity, and a metal-alloy layer havingexcellent corrosion resistance may be provided for improving durability.For example, hard chromium plating may be provided on the surface.Because the roll surface having excessive roughness deteriorates thewettability of the roll with the alloy melt, the surface must befinished by using abrasive paper to a sufficiently smooth surface havingan average surface roughness of ⅓ or less of the ribbon thickness.

After setting such as sample filling, polishing of the roll, and thelike, the chamber is evacuated to 10⁻² torr by a vacuum pump, and aninert gas is introduced into the chamber to a desired pressure. As theinert gas, Ar, He, or the like may be used.

After the desired atmosphere is obtained, the content of the nozzle ismelted to obtain the alloy melt, which is then jetted through theorifice at the bottom of the nozzle.

For jetting, a preferred method comprises jetting the inert gas into thespace above the melt in the nozzle under an appropriate pressure (Pi),as schematically shown in FIG. 1. Specifically, a discharge device forthe inert gas is provided on the upper portion of the nozzle through asolenoid valve so that the pressurized gas in the discharge device isdischarged by opening the solenoid valve with timing for jetting tospray the alloy melt. The substantial injection pressure Pi of the alloymelt is a difference between the pressure of the inert gas in thedischarge device and the atmospheric pressure in the chamber.

The alloy melt jetted as described above is rapidly solidified on theroll to form an alloy ribbon. Since the cooling rate in solidificationincreases as the rotational speed of the roll increases, the rotationalspeed of the roll must be appropriately set to obtain the desired metalstructure. In order to obtain good magnetic properties, good magneticproperties may be obtained in an as-spun state (without heat treatment)or heat treatment may be performed after the alloy ribbon is partiallyor entirely made an amorphous structure. In the former method, therotational speed of the roll must be set to an optimum value. In thelatter method, the rotational speed is set to a value higher than therotational speed at which optimum properties can be obtained in theas-spun state to partially or entirely make the alloy ribbon anamorphous structure in the as-spun state so that after heat treatment,the alloy ribbon is crystallized to obtain magnet characteristics.Although the heat treatment temperature depends upon the alloycomposition, the heat treatment temperature is preferably in the rangeof a temperature immediately above the crystallization temperature to800° C. At a temperature lower than the crystallization temperature,crystallization cannot be achieved, while at a temperature over 900° C.,crystal grains are significantly coarsened, thereby obtainingunsatisfactory magnetic properties.

A magnet powder used for the bonded magnet is obtained by grinding theabove-described magnet alloy ribbon which enables achievement of goodmagnet properties. During grinding, the average particle size of thepowder is preferably 100 μm or less in consideration of moldability ofthe bonded magnet.

The thus-obtained powder is mixed with a thermosetting resin such as anepoxy resin or the like, or a thermoplastic resin such as a nylon resinor the like, and the mixture is molded to obtain the bonded magnet. Asthe molding method, compression molding, injection molding, extrusionmolding, or the like can be used. If required, small amounts of alubricant, an antioxidant, and the like may be added to the resin used.

2) Dimple-like recess

Referring also now to FIG. 2 in-therefore the magnet alloy ribbonproduced by the above-mentioned production method, as a result ofobserving of the surface (referred to as “the roll surface” in thepresent invention) of the alloy ribbon which contacts the metallic rollby a scanning electron microscope (SEM), dispersion of portions recessedin the shape of a dimple 22 (referred to as “the dimple-like recesses”in the present invention) was observed, as shown in FIG. 2. Suchportions are possibly mainly caused by the atmospheric inert gas trappedbetween the alloy melt on the roll and the roll when the melt is rapidlysolidified by jetting on the roll. Such trapping of the gas is possiblymainly due to the viscous flow of the gas generated near the rollsurface with rotation of the roll.

Furthermore, as a result of SEM observation of a broken-out section ofthe ribbon, which was broken, the crystal grain diameter of a normalportion was on the order of several tens nm, while the crystal graindiameter of the main phase of a portion adjacent to the dimple-likerecesses was relatively large, and coarse crystal grains of the order of1 μm were observed in some portions.

The area ratio of the total area of the dimple-like recesses to theentire area of the roll surface was measured by image processing ofphotographs obtained by SEM observation of the roll surface of the alloyribbon. In examples of the present invention which will be describedbelow, the dimple-like recesses in at least ten photographs obtained bySEM observation at a magnification of several tens were first observedby using a contrast difference of an image, and the areas of thedimple-like recesses were converted to a number of pixels to calculatean area ratio. The area ratios of the ten photographs were averaged toobtain a value of the area ratio of the alloy ribbon.

The correlation between the area ratio of the dimple-like recesses andthe magnetic properties of the magnet alloy ribbon was examined indetail. As a result, in the magnet alloy ribbon in which the area ratioof the dimple-like recesses was over 25%, all of coercive force,remanence, and residual magnetic flux density deteriorate to exhibitonly low magnetic properties. In the magnet alloy ribbon having an arearatio of less than 3%, the heat conductivity between the roll and themagneto alloy ribbon is excessively high, thereby causing a largedifference between the cooling rate of the roll surface and the coolingrate of the opposite surface (referred to as “the free surface” in thepresent invention), which does not contact the roll. Therefore,variations in the crystal gain diameter in the roll surface and the freesurface are increased, thereby deteriorating magnetic properties. Also,in the magnet alloy ribbon having an area ratio of less than 3%, therapidly solidified ribbon tends to adhere to the roll because of thehigh adhesion between the roll and the ribbon, thereby deteriorating theyield of the magnet alloy ribbon. In some cases, the roll is rotatedwith the ribbon adhered thereto, and a new melt is jetted on the roll.In such a case, the cooling rate of a portion solidified by newlyjetting on the ribbon, which adheres to the roll, is very low, and thusthe crystal grains are coarsened, thereby deteriorating the magneticproperties of the alloy ribbon obtained.

Since the magnet alloy ribbon has the above-described characteristics,the magnetic properties of the alloy ribbon are reflected in productionof the bonded magnet, and thus the alloy ribbon, in which the area ratioof the dimple-shaped recesses is 3 to 25%, is preferably used.

In consideration of the area of each of the dimples present in the rollsurface, the total area ratio of the dimple-like recesses each having anarea of over 2000 μm² is preferably lower than 5%. As a result of thesame image analysis as described above, the presence of the dimple-likerecesses each having an area of over 2000 μm² not only deteriorates themagnetic properties of the alloy ribbon itself, but also adverselyaffects the reliability of the resultant bonded magnet. Namely, thecorrosion resistance of the bonded magnet deteriorates. This is possiblycaused by the fact that the resin is localized in the dimple-likerecesses having a large area in mixing the magnet powder and the resin,and uniform coating of magnetic powder is thus inhibited.

The depth of the dimple-like recesses also significantly affects themagnetic properties. For measurement of the depth, a laser displacementgage, a micrometer, a capacitance displacement gage, or the like may beused. In the examples of the present invention, which will be describedbelow, for at least 20 individual dimple-like recesses of an alloyribbon of one lot, the distance between the edge of each of dimples andthe bottom thereof was measured as a depth, and the depths were averagedto obtain an average depth d. In order to calculate the averagethickness of the alloy ribbon, the volume was calculated from the weightof the ribbon and the density measured by the Archimedes' method, andthen divided by the width (the average of at least ten measurementsobtained by using a microscope or the like) and the length of theribbon.

When the d/t ratio is higher than 0.5, the magnetic properties of thealloy ribbon significantly deteriorate. In molding the bonded magnet,the porosity is hardly decreased, and the density is hardly increased,thereby deteriorating properties. In addition, the resin isinsufficiently adhered to the dimple portions, thereby adverselyaffecting corrosion resistance. When the d/t ratio is less than 0.1, theadhesion between the alloy ribbon and the roll is increased, therebyundesirably causing the same problems as the case of a low area ratio(less than 3%).

Description will now be made of parameters in the production process forobtaining the magnet alloy ribbon having the above-described surfacestate. As described above, trapping of the inert gas is possibly mainlycaused by the viscous gas flow generated near the roll with rotation ofthe roll. Therefore, it is preferable to take a measure for suppressingsuch a viscous flow. The inert gas atmospheric pressure in the chamberhas the greatest effect. As the atmospheric pressure decreases, trappingof the gas decreases, and the area ratio of the dimple-like recessesalso decreases. However, when the atmospheric pressure is excessivelydecreased, the area ratio becomes less than the range (3%) of thepresent invention, thereby deteriorating the magnetic properties, andcausing variations in production of alloy ribbons. In addition, since anoperation is carried out in a state close to a vacuum, variouslimitations occur in the apparatus used, thereby causing the problem ofincreasing the apparatus cost. Other parameters which influence includethe area of the orifice, the melt temperature (viscosity), and the like.

The present invention will be described in further detail below withreference to examples.

EXAMPLE 1

Each of Nd, Fe and Co metals having a purity of 99.9% or more, and aFe-B alloy (B 19 wt %) was weighed, and melted and cast in an Ar gas ina high-frequency induction melting furnace to obtain a round bar masteralloy ingot having a diameter of 10 mm and the compositionNd₁₂Fe_(bal.)Co₅B_(5.5).

About 15 g of sample per lot was cut out from the ingot, and an alloyribbon was produced by such an apparatus as shown in FIG. 1. Each of thecut samples was placed in a quartz tube having a circular orifice of 0.6mm Ø, and a current was passed through a heating coil to melt the samplein an Ar atmosphere. Then, the alloy melt was jetted on a copper rollrotated at 2000 rpm and having a diameter of 200 mm to obtain a magnetalloy ribbon. In producing the alloy ribbon, the Ar gas atmosphericpressure, and the Ar gas injection pressure were changed to obtainribbons of a total of 8 lots.

For the thus-obtained alloy ribbons of 8 lots, the area ratio of thedimple-like recesses present in the roll surface was calculated by imageanalysis of SEM photographs according to the procedure described in theabove embodiment. The magnetic properties of each of the alloy ribbonswere measured by a vibrating sample magnetometer (VSM) with the maximumapplied magnetic field of 1.44 MA/m in a state where the lengthdirection of the ribbon was located in the direction of the appliedmagnetic field. Table 1 shows the results of measurement of the arearatio of the dimple-like recesses and magnetic properties of each of thelots.

TABLE 1 Area ratio of Lot dimple-like iHc (BH)_(max) No. recess (%)(MA/m) (kJ/m³) A1 2.3 Comparative Example 0.64 38.4 A2 3.0 Thisinvention 0.85 124.3 A3 7.8 This invention 0.79 140.5 A4 11.2 Thisinvention 0.84 138.2 A5 19.8 This invention 0.78 135.9 A6 25.0 Thisinvention 0.70 125.1 A7 27.2 Comparative Example 0.35 81.1 A8 35.1Comparative Example 0.28 52.8

This table indicates that good magnetic properties are obtained in therange of area ratios of 3 to 25%, and magnetic properties deteriorateoutside this range.

Several alloy ribbons were formed by the same method as described aboveusing an ingot having each of the compositions shown in Table 2 at aroll rotational speed of 2000 rpm.

TABLE 2 Composition A Nd₁₂Fe_(bal.)Co₅B_(5.5) Composition BNd_(4.5)Fe_(bal.)Co₅B_(5.5) Composition C Nd_(8.5)Fe_(bal.)B_(5.5)

Each of the alloy ribbons was ground by a kneader to form a powder,which was the mixed with 1.8 wt % of epoxy resin, and molded by a pressunder a pressure of 6 ton/cm² to produce a bonded magnet of 10 mm Ø×7 mmt. The magnetic properties of the thus-obtained bonded magnets weremeasured in a maximum applied magnetic field of 2 MA/m by a DC recordingflux meter. Table 3 shows the area ratio of dimple-like recesses andmagnetic properties of each of the alloy ribbons. This invention andcomparative examples were discriminated according to the area ratio.

TABLE 3 Lot Area iHc (BH)_(max) Composition No. ratio (%) (MA/m) (kJ/m³)Composition A BM-Aa This invention 9.8 0.89 110.2 BM-Ab This invention14.7 0.83 105.9 BM-Ac Comparative 32.4 0.38 43.5 Example Composition BBM-Ba This invention 4.8 0.39 78.3 BM-Bb This invention 20.4 0.35 72.6BM-Bc Comparative 2.6 0.18 10.3 Example BM-Bd Comparative 26.7 0.09 20.4Example Composition C BM-Ca This invention 8.2 0.61 122.1 BM-Cb Thisinvention 24.3 0.64 128.2 BM-Cc Comparative 40.2 0.26 32.4 Example

This table indicates that good magnetic properties can be achieved bythe bonded magnet formed by using the alloy ribbon having dimple-likerecesses at an area ratio in the range of the present invention.

EXAMPLE 2

A magnet alloy ribbon was produced by using a sample cut off from theingot having the composition C shown in Table 2. The roll material, andthe rotational speed were the same as Example 1, and the otherconditions including the injection conditions, atmospheric conditions,etc. were changed to obtain magnetic alloy ribbons of a total of 6 lots.For each of the thus-obtained alloy ribbons, the area ratio ofdimple-like recesses each having an area of 2000 μm² or more wasmeasured by image analysis.

Then, each of the alloy ribbons was ground to form a magnet powder,which was then mixed with 1.8 wt % of epoxy resin and compression-moldedunder a pressure of 6 ton/cm² to obtain a bonded magnet of 10 mm×7 mm t.The magnetic properties of each of the thus-obtained bonded magnets weremeasured by a DC reading flux meter with a maximum applied magneticfield of 2 MA/m. Also corrosion resistance of each of the magnets wasevaluated by a constant-temperature-constant-humidity test at 60° C. and95% RH for 500 hours. The presence of rust on the surfaces was visuallyobserved.

Table 4 shows the results of the area ratio of dimple-like recesses eachhaving an area of 2000 m² or more, magnetic properties, and corrosionresistance of each of the alloy ribbons. In regard to evaluation ofcorrosion resistance, a magnet causing no rust is marked with ◯, and amagnet causing rust is marked with x.

TABLE 4 Area ratio (BH)_(max) Corrosion Lot No. (%) iHc (MA/m) (kJ/m³)resistance BM-Ce 0 0.59 121.9 ∘ BM-Cf 1.2 0.63 125.1 ∘ BM-Cg 2.8 0.65119.2 ∘ BM-Ch 5.0 0.55 120.7 ∘ BM-Ci 6.3 0.48 85.4 x BM-Cj 10.2 0.2451.3 x

This table indicates that a bonded magnet having good corrosionresistance and magnetic properties can be obtained from an alloy ribbonhaving dimple-liked recesses each having an area of 2000 μm² or more atan area ratio of 0 to 5%.

EXAMPLE 3

A round bar-shaped master alloy ingot having the composition(Composition D) Nd₁₁Fe_(bal.)Co₈B_(6.5)V_(1.5) and a diameter of 10 mm Øwas obtained by the same method as Example 1.

A sample of about 15 g per lot was obtained from this ingot, and thenplaced in a quartz tube having a circular hole orifice of 0.6 mm Øprovided at the bottom thereof. A current was passed through a heatingcoil to melt the sample in an Ar atmosphere, and the resultant melt wasjetted on a copper roll having a diameter of 200 mm and rotating at 4000rpm to obtain a magnet alloy ribbon. In producing an alloy ribbon,injection conditions and atmospheric conditions were changed to obtainalloy ribbons of a total of 8 lots. For each of the thus-obtainedribbons, the d/t ratio of the average depth to the average thickness wasmeasured by the method described above in the embodiment.

As a result of X-ray diffraction of the alloy ribbons, all diffractionpeaks were broad peaks. It was thus confirmed that the structure of eachof the alloy ribbons is partially amorphous. After heat treatment in Arat 650° C. for 10 minutes, the magnetic properties of these ribbons weremeasured by the same method as Example 1.

Table 5 shows the d/t value and magnetic properties of each of the alloyribbons.

TABLE 5 Lot No. d/t iHc (MA/m) (BH)_(max) (kJ/m³) D1 0.05 ComparativeExample 0.68 77.8 D2 0.10 This invention 0.81 133.2 D3 0.18 Thisinvention 0.83 136.0 D4 0.28 This invention 0.79 131.5 D5 0.36 Thisinvention 0.82 128.3 D6 0.50 This invention 0.72 125.1 D7 0.55Comparative Example 0.35 85.4 D8 0.64 Comparative Example 0.28 41.9

This table indicates that good magnetic properties can be obtained by analloy ribbon having a d/t ratio of 0.1 to 0.5.

Several alloy ribbons were formed by using an ingot having each of thecompositions shown in Table 6 at a roll rotational speed of 4000 rpm,with the injection conditions and atmospheric conditions changed. Thed/t ratio of each of the ribbons was measured.

TABLE 6 Composition E Nd₁₃Fe_(bal.)B_(5.5)Nb_(1.0) Composition FNd_(9.0)Fe_(bal.)B_(6.0)Co_(1.0)

After heat treatment at a temperature higher than the crystallizationtemperature of each of the compositions for 10 minutes, each of theribbons was ground by a kneader to form a powder which was then mixedwith 1.8 wt % of epoxy resin, and compression-molded under a pressure of6 ton/cm² to obtain a bonded magnet of 10 mm Ø×7 mm t. The magneticproperties of each of the bonded magnets were measured by a DC readingflux meter in a maximum applied magnetic field of 2 mA/m. Also corrosionresistance of each of the magnets was evaluated by aconstant-temperature-constant-humidity test at 60° C. and 95% RH for 500hours. The presence of rust on the surface was determined by visualobservation.

Table 7 shows the results of measurement of the area ratio, magneticproperties, and corrosion resistance of each of the alloy ribbons. Inthe table, in evaluation of corrosion resistance, a magnet causing norust is marked with ◯, and a magnet causing rust is marked with x.

TABLE 7 Lot area (BH)_(max) Corrosion Composition No. ratio (%) (kJ/m³)resistance Composition E BM-Ea This 4.8 65.0 ∘ invention BM-Eb This 20.463.2 ∘ invention BM-Ec Comparative 2.6 39.8 x Example BM-Ed Comparative26.7 41.2 x Example Composition F BM-Fa This 8.2 120.7 ∘ invention BM-FbThis 24.3 118.3 ∘ invention BM-Fc Comparative 40.2 50.1 x Example

This table reveals that a bonded magnet having good corrosion resistanceand magnetic properties can be obtained from an alloy ribbon having a anarea ratio in the range of the present invention.

What is claimed is:
 1. A magnet alloy ribbon obtained by jetting a rareearth element-transition metal-boron alloy melt on a rotating metallicroll to rapidly solidify the alloy melt, wherein a total area ratio ofdimple recesses after solidification, which are present in a surface ofthe ribbon in contact with the roll during solidification, is 3 to 25%.2. A magnet alloy ribbon obtained by jetting a rare earthelement-transition metal-boron alloy melt on a rotating metallic roll torapidly solidify the alloy melt, wherein a total area ratio of dimplerecesses, which are present in a surface of the ribbon in contact withthe roll during solidification and each of which has an area of 2000 μm²or more, is 0 to 5%.
 3. A magnet alloy ribbon obtained by jetting a rareearth element-transition metal-boron alloy melt on a rotating metallicroll to rapidly solidify the alloy melt, wherein a d/t ratio of anaverage depth (d) of dimple recesses after solidification to an averagethickness (t) of the alloy ribbon, which recesses are present in asurface of the ribbon in contact with the roll during solidification, is0.1 to 0.5.
 4. A resin bonded magnet obtained by grinding a magnet alloyribbon before or after heat treatment, which is obtained by jetting arare earth element-transition metal-boron alloy melt on a rotatingmetallic roll to rapidly solidify the alloy melt, to form a powder;mixing the powder and a resin into a mixture; and then molding themixture; wherein a total area ratio of dimple recesses aftersolidification, which are present in a surface of the ribbon in contactwith the roll during solidification, is 3 to 25%.
 5. A resin bondedmagnet obtained by grinding a magnet alloy ribbon before or after heattreatment, which is obtained by jetting a rare earth element-transitionmetal-boron alloy melt on a rotating metallic roll to rapidly solidifythe alloy melt, to form a powder; mixing the powder and a resin into amixture; and then molding the mixture; wherein a total area ratio ofdimple recesses, which are present in a surface of the ribbon in contactwith the roll during solidification and each of which has an area of2000 μm² or more, is 0 to 5%.
 6. A resin bonded magnet obtained bygrinding a magnet alloy ribbon before or after heat treatment, which isobtained by jetting a rare earth element-transition metal-boron alloymelt on a rotating metallic roll to rapidly solidify the alloy melt, toform a powder; mixing the powder and a resin into a mixture; and thenmolding the mixture; wherein a d/t ratio of an the average depth (d) ofdimple recesses after solidification to an average thickness (t) of thealloy ribbon, which recesses are present in a surface of the ribbon incontact with the roll during solidification, is 0.1 to 0.5.